A number of different electronic identification systems and devices are presently in use, ranging from ubiquitous bar code and magnetic strip systems to silicon chip radio frequency identification (“RFID”) tag systems. Because bar codes and magnetic strips have a very short effective read range and are usually read one at a time, seldom will the reading of one bar code or magnetic strip interfere with the reading of another. Although silicon chip based RFID tag systems have a longer read range than magnetic strip or bar code systems, the tags are relatively expensive and are not widely used (except for limited applications such as toll tags and security systems), which means most such prior art RFID tags will also generally be individually read with little opportunity for one such tag to interfere with another.
Up to the present time, the current state of the art has not addressed the significant signal collision problem caused by simultaneous identification tag responses. As RFID tags become less expensive, RFID tag suppliers are working hard on solving the collision problem. Because of the nature of a semiconductor tag the approach is quite different than the approach taken for SAW tags. The semiconductor approach involves a protocol that limits the number of smart tags in a field of view that respond to an interrogation. The term “smart” refers to the fact that an individual semiconductor tag, or group of tags with a common address code, can be commanded to respond or not respond.
The situation is different, however, with respect to identification tags that are based on surface acoustic wave (SAW) technology. Because all SAW tags in a field of view always respond a different approach is needed. With the introduction of inexpensive identification SAW tags the problem has ripened to a point where a solution to signal collisions is required.
The problem is quite apparent when a number of SAW tags are simultaneously interrogated, which will occur frequently because SAW technology permits tags to be read at a relatively long range. With each SAW tag responding to the interrogation signal, it is a certainty that interference problems will occur. The problem can be further illustrated by considering the transmission of an interrogation pulse to a large number of articles, each with its own uniquely coded SAW identification tag, that are stacked on a pallet. Because each SAW identification tag on each article will respond by transmitting its own globally unique coded response signal, it may be difficult to process the mass of signal data to accurately detect and decode each response and reliably identify each article. This kind of code collision problem as well as other interference problems caused by so much data being transmitted at one time must be resolved before the full potential of SAW identification tag technology can be realized.
Accordingly, what is needed in the art is a method to accommodate code collisions caused by simultaneous responses from multiple SAW identification tags.